Electrical thermostats for changing the ambient temperature setting of the thermostat at selected times and thus control the operation of a heating or cooling unit are well known. Customarily, available thermostatic devices of this type are installed in a two-wire, low voltage control circuit in a position remote from the furnace or air conditioner to be controlled. In designing electrical circuits to permit this type of installation and operation without requiring additional wiring to the thermostat many attempts have been made to utilize the low voltage (typically 24V AC) of the control circuit to drive the necessary timing mechanism and to actuate the starting relay or gas valve actuator in the case of a gas furnace, only when the thermostat calls for heat.
In practice, to the inventor's knowledge, all such circuits designed for direct connection into existing twowire control circuits without additional wiring have required either an additional source of power to drive the timing mechanism such as a battery (see co-pending United States Patent Application, Ser. No. 497,151, filed Aug. 13, 1974, now U.S. Pat. No. 3,948,441 which issued Apr. 6, 1976 and assigned to the same Assignee as this application) or some form of means for receiving and storing energy from the two-wire control circuit during periods when the timing mechanism does not require electrical energy (see the earlier invention described in co-pending United States Patent Application, Ser. No. 706,667, filed July 19, 1976 and assigned to the same Assignee as this application).
In the above devices, which represent the current state of the art to the inventor's knowledge, each requires a timing mechanism with its own time base that is independent of the frequency of alternating current in the low voltage control circuit. While various methods have been used, such as spring-wound clock mechanisms and battery-operated clock movements, none, to the inventor's knowledge, has heretofore been devised that makes it possible at low cost to drive a sychronous motor for a timing mechanism directly from an existing two-wire thermostat without any additional wiring being required.